1. Field of the Invention
The present invention relates to a variable attenuator for use in a lithographic apparatus. The invention also relates to a lithographic apparatus and a method for manufacturing a device. In particular, the invention relates to a variable attenuator for regulating pulse energy density.
2. Background of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticule, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It is important to control the radiation dose applied to the substrate. In the Deep Ultraviolet (DV) wavelength region, such as for example 248 nm, 193 nm, or 157 nm, radiation is usually applied using a pulsed exciter laser. In many cases the fluctuation in the energy of individual laser pulses is too large, or the total number of pulses in the slit is too low, to guarantee accurate exposures. This is a particular problem for low doses. In order to overcome this problem, a variable attenuator (VAT) is generally placed in the path of the beam to control the radiation intensity at the wafer stage. A typical variable attenuator is able to reduce the pulse energy to a value between 7% and 90% of its initial value. By decreasing the intensity of each pulse the total number of pulses required is increased, and deviations between individual pulses can be averaged out more effectively.
Known variable attenuators comprise a transmissive substrate (e.g. a quartz plate) coated with an “angle-sensitive” layer whose transmission is dependent on the angle of radiation incident upon it. The plate can be rotated to vary the transmission of the attenuator in a range of, typically, approximately 7-90%.
As mentioned above, two exposure types can be distinguished: static exposures (“stepper” apparatus) and scanning exposures. The way the variable attenuator is used during these types of exposures is different. In the case of static exposures the variable attenuator switches between its maximum and minimum transmission points, whereas during scanning exposures the transmission can be anywhere between these extremes.
A static exposure starts with the variable attenuator at its maximum transmission. As the integrated dose increases, the dose control algorithm evaluates whether the next pulse (including its inherent energy error) will still fit within the dose ready window. If so, the next pulse will be fired. If not, the exposure is briefly interrupted to move the variable attenuator from the maximum to the minimum transmission. From this point on, the exposure is finished with low-energy pulses until the integrated dose fits within the dose ready window.
Since not all parts of the die are exposed simultaneously during a scanning exposure, the variable attenuator cannot be used in the same way, as different pulses affect different parts of the die. Therefore, the transmission of the variable attenuator may not be changed during scanning exposures. A required set point is calculated and set before the exposure starts, depending on the scan time (i.e. wafer stage speed), laser frequency, nominal pulse energy, and number of pulses. As the set point of the variable attenuator is fixed during the exposure, a dose control algorithm manipulates the energy of each laser pulse.
As mentioned above, currently available attenuators are capable of providing a transmission range of approximately 7-90%. It would be desirable to provide an attenuator capable of transmitting over a range of 0-100%. At the top end of this range, the additional transmission could increase the throughput. At the lower end of the range, the greater attenuation would enable low doses to consist of more pulses, improving the averaging between pulses.
Furthermore, variable attenuators currently in use suffer from in homogeneity problems with the angle-sensitive coating. This results in a change in the transmission of the attenuator as the beam moves over its surface. This change should be compensated for by the laser changing the energy provided in each pulse. Since the available range of energy per pulse is limited, this can lead to dose errors at the wafer level.